This invention relates to a nitride compound semiconductor light emitting element and its manufacturing method. More particularly, the invention relates to a nitride compound semiconductor light emitting element having a high confinement efficiency for light and carrier and suppressing leak current, and a method for manufacturing same.
Nitride compound semiconductors are of direct-transition type and are capable of highly efficient radiation recombination in almost all composition range. Their transition energy widely ranges from 1.89 to 6.2 eV, depending on the composition. Because of these advantageous characteristics, light emitting elements using nitride compound semiconductors are expected to be useful elements for intense emission especially in short wavelength range in various fields of application.
In the present application, the term "nitride compound semiconductor" pertains to any semiconductor which can be expressed by the chemical formula B.sub.x In.sub.y Al.sub.z Ga.sub.1-x-y-z N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1, x+y+z.ltoreq.1) with any values of the mole fractions x, y and z in their respective ranges. For example, InAlN (x=0, y=0.4, z=0.6) has been identified as one the of nitride compound semiconductors.
Conventional nitride semiconductor light emitting devices, in general, have a multi-layered structure made by sequentially growing flat layers of various nitride compound semiconductor crystals on a flat substrate of sapphire, or the like. For example, a double-heterostructure representative as a semiconductor light emitting element is made by sequentially stacking a cladding layer, active layer and cladding layer on a substrate via an appropriate buffer layer. Such a double-heterostructure can effectively confine injected carriers and light generated in the active layer, and is indispensable especially for fabricating a high-performance semiconductor laser.
A high-performance semiconductor laser cannot be realized only by using a double-heterostructure. It is also necessary to confine the current and light in the active layer with high efficiency. However, conventional light emitting elements using nitride compound semiconductors could not ensure such a high efficiency in confinement of current and light. Details about this problem are explained below.
For efficient confinement of current, the active layer stacked as a flat layer must be processed to form a structure for concentrating the current. The most popular process used for the purpose relies on processing the active layer into a narrow mesa configuration and burying it in a layer or layers made by subsequent crystal growth. In the case of forming light emitting devices with nitride compounds, if dry etching is used for processing the active layer into the form of a stripe in nitride light emitting elements, it often damages the etched surface and fails to realize good current characteristics. Wet etching is less harmful to the etched surface, but there has been no appropriate wet etching process suitable for nitride compound semiconductors.
Another problem with nitride compound semiconductors lies in the difficulty in growing layers for burying the mesa-shaped active layer. Even if the mesa-shaped active layer can be buried incompletely, it tends to result in irregular growth and to cause leakage of current.
Under circumstances it is difficult to effectively confine current in conventional light emitting elements using nitride compound semiconductors.
Additionally, confinement of light is also difficult in conventional light emitting element using nitride compound semiconductors. That is, the refractive index of the active layer must be as high as possible relative to the cladding layer to ensure efficient confinement of light in the active layer. To realize a high refractive index of a nitride compound semiconductor, the mole fraction of indium in the nitride compound semiconductor must be high. However, as the mole fraction of indium increases, the crystallographic quality of the semiconductor degrades unacceptably to maintain characteristics required for the active layer.
Moreover, in the case of metal-organic chemical vapor deposition (MOCVD) widely used for growth of nitride compound semiconductors, the temperature must be low during crystal growth to ensure a high mole fraction of indium. However, such a low growth temperature causes further deterioration in crystalline quality and degrades various characteristics of the resulting light emitting element. That is, conventional techniques cannot not realize highly efficient laser oscillation and emission of shorter wavelengths with nitride semiconductor light emitting devices by increasing the mole fraction of indium.